The causes of congenital hypothyroidism include athyrosis, hypoplasia, ectopic hypoplasia and maternal autoimmunization of the thyroid gland and inadequate synthesis, delivery and peripheral effect of thyroid hormones. With deficient thyroid hormone production or effect in patients before the age of two years, irreversible mental retardation develops unless the diagnosis and appropriate replacement thyroid therapy are instituted within the first few months of life. The incidence of congenital hypothyroidism is unknown, but is indirectly estimated to occur between one in five thousand to one in ten thousand live births. When there is deficient thyroid hormone production or effect, serum thyrotropin (TSH) levels increase in an effort to augment the thyroid hormone (thyroxin and triiodothyronine, T4 and T3) synthesis and release. With the improved techniques to very accurately determine serum thyrotropin levels by radioimmunoassay, the utilization of these techniques should provide the development of an effective, simple accurate screening test for congenital hypothyroidism in the neonate in order to achieve the following objectives: 1. provide prompt diagnosis and therapy to prevent or modify almost completely the inevitable subsequent development of mental retardation in untreated infants; 2. directly determine the incidence of congenital hypothyroidism.